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The present invention relates to organic light emitting devices
(OLED) and more particularly to determining defects in such devices.
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An organic light-emitting device, also referred to as an organic
electroluminescent device, can be constructed by sandwiching two or more
organic layers between first and second electrodes.
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In a passive matrix organic light-emitting device of conventional
construction, a plurality of laterally spaced light-transmissive anodes, for example
indium-tin-oxide (ITO) anodes are formed as first electrodes on a light-transmissive
substrate such as, for example, a glass substrate. Two or more
organic layers are then formed successively by vapor deposition of respective
organic materials from respective sources, within a chamber held at reduced
pressure, typically less than 10-3 Torr. A plurality of laterally spaced cathodes are
deposited as second electrodes over an uppermost one of the organic layers. The
cathodes are oriented at an angle, typically at a right angle, with respect to the
anodes.
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Such conventional passive matrix organic light-emitting devices
are operated by applying an electrical potential (also referred to as a drive voltage)
between an individual row (cathode) and, sequentially, each column (anode).
When a cathode is biased negatively with respect to an anode, light is emitted
from a pixel defined by an overlap area of the cathode and the anode, and emitted
light reaches an observer through the anode and the substrate.
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In an active matrix organic light-emitting device, an array of
anodes is provided as first electrodes by thin-film transistors (TFTs) which are
connected to a respective light-transmissive portion. Two or more organic layers
are formed successively by vapor deposition in a manner substantially equivalent
to the construction of the aforementioned passive matrix device. A common
cathode is deposited as a second electrode over an uppermost one of the organic
layers. The construction and function of an active matrix organic light-emitting
device is described in commonly-assigned US-A-5,550,066, the disclosure of
which is herein incorporated by reference.
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Organic materials, thicknesses of vapor-deposited organic layers,
and layer configurations, useful in constructing an organic light-emitting device,
are described, for example, in US-A-4,356,429; US-A-4,539,507; US-A-4,720,432;
and US-A-4,769,292, the disclosures of which are herein incorporated
by reference.
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In color or full-color organic electroluminescent (EL) displays
having an array of colored pixels such as red, green, and blue color pixels
(commonly referred to as RGB pixels), precision patterning of the color-producing
organic EL media is required to produce the RGB pixels. The basic
organic EL device has in common an anode, a cathode, and an organic EL
medium sandwiched between the anode and the cathode. The organic EL medium
can consist of one or more layers of organic thin films, where one of the layers is
primarily responsible for light generation or electroluminescence. This particular
layer is generally referred to as the light-emitting layer of the organic EL medium.
Other organic layers present in the organic EL medium can provide electronic
transport functions primarily, such as the hole-transporting layer or the electron-transporting
layer. In forming the RGB pixels in a full-color organic EL display
panel, it is necessary to devise a method to precisely pattern the light-emitting
layer of the organic EL medium or the entire organic EL medium.
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Typically, electroluminescent pixels are formed on the display by
shadow masking techniques, such as shown in US-A-5,742,129. The shadow
mask is designed allow a plurality of pixels to be deposited during one deposition
step. By performing multiple deposition steps with different materials with
different emission characteristics, full-color pixels consisting of red, green, and
blue sub-pixels can be produced. Although the shadow masking technique has
been effective, it has several drawbacks. It has been difficult to achieve high
resolution of pixel sizes using shadow masking. Moreover, there are problems of
alignment between the substrate and the shadow mask, and care must be taken that
pixels are formed in the appropriate locations. When it is desirable to increase the
substrate size, it is difficult to manipulate the shadow mask to form appropriately
positioned pixels. A further disadvantage of the shadow-mask method is that the
mask holes can become plugged with time. Plugged holes on the mask lead to the
undesirable result of non-functioning pixels on the EL display. Consequently, it is
desirable to have some way of inspecting the OLED device for problems
originating with the shadow mask.
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It is therefore an object of the present invention to provide a
detection arrangement that is particularly effective for determining defects in size,
shape, location, and light intensity of pixels in one or more OLED devices.
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This object is achieved by a method of determining defects in
OLED devices having a plurality of pixels, each with its own emissive layer,
which are capable of being excited by input light to produce an output color light
response, comprising the steps of:
- a) illuminating one or more OLED devices with light in a
predetermined portion of the spectrum so that the pixel emissive layers are excited
to produce an output color response for each pixel;
- b) capturing an image of the output light produced by the
excited pixels and converting such captured light into a digital image; and
- c) determining size, shape, location, and light intensity in
response to the captured digital pixel and comparing such size, shape, location,
and light intensity with predetermined acceptable size, shape, location, and light
intensity ranges to determine whether there is a defect in the OLED device(s).
-
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The present invention provides the following advantages. It makes
use of the property that the emissive layers of OLED devices can be excited by a
illumination source using a predetermined portion of the spectrum and an image
can be captured from the excited pixels of the OLED device. By operating on the
converted digital image an effective defect detection arrangement can be
achieved. The present invention can be configured to operate in a variety of
environments, including operation within a vacuum or controlled atmosphere
chamber.
- FIG. 1 is a schematic perspective of a passive matrix organic light-emitting
device (OLED) having partially peeled-back elements to reveal various
layers;
- FIG. 2 is a schematic perspective of a manufacturing system
suitable for manufacture of a relatively large number of organic light-emitting
devices (OLEDs) and having a plurality of stations extending from hubs;
- FIG. 3 is a block diagram of a system for practicing the method in
accordance with the present invention;
- FIG. 4A is a detailed arrangement showing the x, y positioning
system for inspection within a vacuum or controlled atmosphere chamber;
- FIG. 4B is an enlarged view of the orientation of the camera,
illumination source, and substrate within a vacuum or controlled atmosphere
chamber;
- FIG. 5A is a detailed arrangement showing the x, y positioning
system for inspection of an encapsulated device;
- FIG. 5B is an enlarged view of the orientation of the camera,
illumination source, and substrate of an encapsulated device;
- FIG. 6 is a flowchart used by the computer shown in FIG. 3 for
determining detected defects in the OLED devices by using step and measure
inspection system, and
- FIG. 7 is a flowchart used by the computer shown in FIG. 3 for
determining detected defects in the OLED devices by using a continuous scanning
inspection system.
-
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The inspection system described herein utilizes the phenomenon
called photoluminescence. Photoluminescence is the process whereby a material
absorbs light energy within a certain wavelength range, and re-emits that light
energy at longer wavelengths. Phosphorescence and fluorescence are two
separate emission pathways collectively termed photoluminescence. The
absorbed light is called excitation light, and the resulting luminescent light is
called emission light. The wavelengths of absorption and emission are dependent
upon the specific composition of the material; in the case of the organic materials
deposited in an OLED display, the emission wavelengths from photoluminescence
are very similar to the wavelengths generated by electroluminescence, the
underlying process for normal display usage. Because the excitation wavelengths
depend upon the absorption characteristics of the material to be inspected, a
predetermined portion of the spectrum can be chosen for best excitation. This
makes inspection via photoluminescence particularly useful in assessing the
quality of the light-emitting layers prior to assembling a completed display device.
The emitted light from the excited pixels can be captured by a camera into a
digital image and subsequently analyzed and evaluated for various quality criteria
such as device pixel size, shape, location, and emitted light intensity. In addition,
by using a color camera for the image capture, an analysis of the color of the
emitted light can be performed and compared to acceptable color criteria.
Different colored light produced by different device pixels can be captured by the
camera. By specifying predetermined acceptable limits for these quality criteria, a
device can be categorized as to whether or not it is defective. This categorization
can be performed automatically by a computer program or an output image can be
generated from the criteria and a user can inspect the output image to determine
the categorization.
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Turning to FIG. 1, a schematic perspective of a passive matrix
organic light-emitting device (OLED) 10 is shown having partially peeled-back
elements to reveal various layers.
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A light-transmissive substrate 11 has formed thereon a plurality of
laterally spaced first electrodes 12 (also referred to as anodes). An organic hole-transporting
layer (HTL) 13, an organic light-emitting layer (LEL) 14, and an
organic electron-transporting layer (ETL) 15 are formed in sequence by a physical
vapor deposition, as will be described in more detail hereinafter. A plurality of
laterally spaced second electrodes 16 (also referred to as cathodes) are formed
over the organic electron-transporting layer 15, and in a direction substantially
perpendicular to the first electrodes 12. An encapsulation or cover 18 seals
environmentally sensitive portions of the structure, thereby providing a completed
OLED 10.
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Turning to FIG. 2, a schematic perspective of a manufacturing
system 100 is shown which is suitable for manufacture of a relatively large
number of organic light-emitting devices using automated or robotic means (not
shown) for transporting or transferring substrates or structures among a plurality
of stations extending from a buffer hub 102 and from a transfer hub 104. A
vacuum pump 106 via a pumping port 107 provides reduced pressure within the
hubs 102, 104, and within each of the stations extending from these hubs. A
pressure gauge 108 indicates the reduced pressure within the system 100. The
pressure can be in a range from about 10-3 to 10-6 Torr.
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The stations include a load station 110 for providing a load of
substrates or structures, a vapor deposition station 130 dedicated to forming
organic hole-transporting layers (HTL), a vapor deposition station 140 dedicated
to forming organic light-emitting layers (LEL), a vapor deposition station 150
dedicated to forming organic electron-transporting layers (ETL), a vapor
deposition station 160 dedicated to forming the plurality of second electrodes
(cathodes), an unload station 103 for transferring structures from the buffer hub
102 to the transfer hub 104 which, in turn, provides a storage station 170, and an
encapsulation station 180 connected to the hub 104 via a connector port 105.
Each of these stations has an open port extending into the hubs 102 and 104,
respectively, and each station has a vacuum-sealed access port (not shown) to
provide access to a station for cleaning, replenishing materials, and for
replacement or repair of parts. Each station includes a housing which defines a
chamber.
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Turning to FIG. 3, a block diagram is shown which depicts the
general arrangement of the system components in accordance with the present
invention. Motion controller 224 directs the movement of motorized x-axis
translation slide 232 and motorized y-axis translation slide 234, and optionally
triggers camera 236 and/or illumination source controller 238. Measurement
computer 226 initializes motion controller 224, camera 236, and illumination
source 238. Measurement computer 226 acquires image from camera 236, and a
computer program automatically processes and analyzes the image, then outputs a
set of measurements that is received by database/process control computer 228.
Arrows show the direction of data flow 230 between the system components. To
image the entire area of substrate 11 with sufficient resolution for analysis, it is
necessary to acquire multiple images and translate the position of the camera 236
and/or the substrate 11 between each acquisition.
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Two preferred translation arrangements are depicted in FIG. 4 and
FIG 5 and are described below. In both arrangements, the y-axis translation slide
234 translates camera 236 across one substrate axis, here chosen to be the narrow
substrate dimension, and its motion can be stepped or continuous. For the
remaining axis, here chosen to be the x-axis, the translation action differs between
the two arrangements. X-axis translation slide 232 translates (preferably with
stepped motion) the camera 236 in the first arrangement and substrate 11 in the
second arrangement. In either arrangement, the database/process control
computer 228 maintains a historical archive of the measurements, and provides
tracking, trending, and alarming functions based on the measurement results.
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Turning to FIG. 4A, one of the preferred embodiments of the system is
shown. In this embodiment, substrate 11 is located with the vacuum chamber of
the OLED manufacturing system 100 or within a controlled atmosphere chamber
outside of the vacuum chamber. Inspection of the substrate 11 is accomplished
with the camera 236 located outside the vacuum chamber by imaging through
inspection window 248. An illumination system includes a plurality of elements
which will now be discussed. They are flexible light guides 244, light guide
mount 258, illumination optics 250 which are located outside the vacuum
chamber. An alternate configuration can locate the camera and/or illumination
source within the vacuum chamber. The camera 236 and illumination optics 250
are translated along inspection window 248 using y-axis translation slide 234 and
flexible light guides 244. The direction of translation along the y-axis is indicated
by arrow 240. As an alternative to light guides 234, a smaller illumination source
such as an LED or fluorescent tube ringlight can be translated along with camera
236. After scanning the desired locations along the y-axis, the substrate is
translated along the x-axis as shown by arrow 254, and then the y-axis scan is
repeated to inspect new areas as desired.
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It should be noted that many orientations of the illumination source
and camera with respect to the substrate 11 are possible; for example, the substrate
11 can be inverted, or the camera placed on the opposite side of the substrate from
the illumination source, or both.
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FIG. 4B shows an enlarged view of the inspection system shown in
FIG. 4A. Illumination of the substrate 11 within vacuum 262 is achieved by
conducting the illumination light 264 through light guide 244, illumination optics
250 and inspection window 248. The light passes through substrate 11 and a
portion is absorbed by the organic materials the device pixels 266, 268, 270.
Illumination optics 250 can include components such as lenses and filters
necessary to improve the quality of the illumination light 264. An alternate
orientation would be where the inspection is performed from the same side of the
substrate 11 as the organic layers, permitting inspection on non-transmissive
substrates. For the purposes of this inspection, the substrate 11 can be formed of
any material, including glass, polymer, and silicon.
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Excitation with light in the ultraviolet region offers the potential of
inducing emissions of all colors in the visible portion of the spectrum. A
wavelength of 365 nm is preferred. The excitation light is provided by an
illumination source, which can be any of a variety of sources such as fluorescent
tube lamps, ring lights, diffuse lighting fixtures, light emitting diodes, and
mercury or xenon arc lamps or flashlamps. Upon emission, a portion of the light
passes back through substrate 11 and inspection window 248, and is captured by
camera 236. The camera lens 252 can provide magnification, allowing inspection
of very small features. A filter 260 can be employed on camera lens 252 to limit
the range of wavelengths reaching the camera imager.
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Another inspection arrangement is shown in FIG. 5A. In this embodiment,
a preferably encapsulated OLED substrate can be inspected outside the vacuum
chamber, typically at an inspection station. To reduce the space occupied by the
system, the camera 236 is translated (rather than the substrate 11) in both x- and y-directions
(indicated by translation arrows 240, 276) using translation slides 232,
234, and secondary x-axis translation slide 278 in order to scan the substrate area.
Secondary x-axis translation slide 278 is optional, and if present it may not be
motorized depending on the weight of the mounted components and the degree of
structural stability provided by slides 232 and 234. To accommodate a short lightguide
length, the illumination source 238 can be mounted to the outer housing of
y-axis slide 234 in a fixed position, and translated along the x-axis by slide 232.
FIG. 5B shows an enlarged view of the inspection arrangement. This arrangement
is similar to FIG. 4B, but here the substrate 11 is shown as encapsulated by cover
18 and is not depicted within the vacuum chamber. Another embodiment of this
invention holds the camera stationary while moving the substrate, which may be
advantageous if the larger physical space occupied by the system is acceptable.
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In accordance with the present invention, a measurement and
analysis process can be incorporated to produce a complete system for inspecting
a substrate containing one or more OLED devices. Ideally an image of the entire
substrate would be acquired in a single camera image, but this it is generally not
possible for larger substrates as the area that can be inspected with sufficient
resolution for analysis is limited by the available resolution of camera imagers.
Consequently, the inspection system must be capable of imaging a portion of the
substrate surface and/or the entire substrate surface by sampling the substrate via
individual images from different locations. These images are termed image tiles,
and can optionally be assembled to produce a mosaic image of the entire substrate
(or portion thereof).
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A computer program is employed to automatically analyze the
images and perform the desired measurements. Two measurement scenarios are
described here, although others are possible. The sequence of the steps has been
arranged in a logical order, but all steps do not necessarily have to be performed in
the described sequence. The first scenario is a stepped measurement process in
which the movement is stopped during image acquisition. The second scenario is
a continuous scan process in which the camera and substrate are translated relative
to each other during image acquisition. In this second scenario, to achieve good
image quality, motion blur in the image must be prevented by shuttering or
strobing the camera and/or illumination source. Either scenario can employ a
continuous or strobed illumination source.
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FIG. 6 shows the steps in a stepped measurement process in
accordance with the invention. This process generally follows the order of step to
next position, stop, measure, and repeat. The substrate 11 is placed on an
inspection platform or translation stage (step 280) and the illumination source
energized if it is continuous (step 282). The substrate 11 can be optionally
mounted in a frame for better support and handling. The substrate 11 or camera
236 is moved to the starting position (step 284) and the first photoluminescent
image is acquired by the camera 236 (step 286) and transferred to measurement
system computer 226 for subsequent analysis. If the illumination source 238 is
not continuous, then the illumination source 238 is strobed during the image
acquisition by the camera 236 to minimize motion blur in the acquired image. At
this point in the process, there are two branches in the flow of operations,
consisting of an analysis branch ( steps 288, 290, 292, 294, and 296) and a motion
branch (steps 298, 300) which can be run sequentially or in parallel if desired. In
the analysis branch, the image can first be processed to improve image quality
(step 288). The image is then thresholded to produce a binary image in which all
image pixels are either white or black (step 290). The thresholding is performed
such that all objects of interest in the image become white, while all background
areas become black. For this analysis, the objects of interest constitute the
photoluminescent emissions from areas of coated organic materials, in particular
emissions from the sub-pixels of the OLED device. Particle analysis techniques
(also called blob analysis techniques) are then applied to locate the device sub-pixels
(step 292) and measure the parameters of the sub-pixels (step 294).
Parameters of interest include but are not limited to size, shape, location, intensity,
and color. These measurements are then recorded (step 296) within the
measurement system computer 226. In the motion branch of the process, the
substrate or camera is moved to the position of the next image tile (step 298) and
then stopped (step 300) to await acquisition of the next image tile. After both
analysis and motion branches have completed, the progress is evaluated to
determine whether the all image tiles have been acquired (step 302). If there are
additional image tiles to be acquired, the process is repeated starting with step
286. If all image tiles have been acquired, a comparison is performed to see if the
measurements made on this substrate fall within predefined limits for these
measurements (step 304). Those measurements falling outside the predefined
limits are defined to be defects. Finally, the defect data is stored and optionally
displayed (step 306) pending archiving by the database / process control computer
228. The process terminates with step 308.
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The continuous scan measurement arrangement is described in
FIG. 7. Continuous measurement is made possible by strobing the excitation light
using a pulsed or shuttered illumination source to minimize motion blur in the
acquired image. Alternatively the emission light can be shuttered with either a
mechanical or electronic shutter to minimize motion blur in the acquired image.
The substrate 11 is first placed on the translation mechanism (step 310). The
substrate can be optionally mounted in a frame for better support and handling.
The substrate 11 and/or camera 236 is then moved to the starting position (step
312). A continuous scan is then begun along one translation axis (step 314). The
position is monitored by motion controller 224, and at the location for the next
desired image tile, a trigger signal is generated for the camera 236 and/or
illumination source 238 (step 316). Upon receiving this signal, the camera 236
acquires the desired image tile (step 318). Motion blur in the image is prevented
by either strobing the illumination source 238 for a sufficiently short time, or by
shuttering the camera 236 and/or illumination source 238 for a similarly short
time. Upon completed image acquisition, the steps of the previously described
analysis branch ( steps 288, 290, 292, 294, and 296) are performed. Upon
completion of these steps, the motion controller 224 is polled to determine if the
current scan line has been completed (step 330). If not complete, the image
acquisition and analysis steps are repeated starting at step 316. If the scan line is
complete, the current motion is stopped and the scan line counter is incremented
(step 332). The value of the scan line counter is then compared to the desired
number of scan lines (step 334). If the desired number of scan lines has not yet
been achieved, the acquisition and analysis of the next scan line is begun by
repeating the process starting at step 314. Once the desired number of scan lines
has been reached, the process continues as previously described in steps 304 and
306, and terminates with step 308.
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With the inspection system described above, it is also possible to
inspect shadow masks for photoluminescent material. This usage can be of
interest for detecting residual organic material after performing a mask cleaning
process. If a shadow mask was properly cleaned, it can be assumed that there
would be little or no detected photoluminescent material. Detected
photoluminescent particles can be compared to allowable tolerances similar to the
analysis previously described. The residual material would be detectable whether
it was on the mask surface or within the holes of the shadow mask. In addition, if
the shadow mask was placed on a fluorescent background, the holes themselves
can be inspected with or without the presence of residual organic material.
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Other features of the invention are included below.
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The method including different colored light from different pixels
in the OLED device.
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The method wherein the illumination source produces ultraviolet
light which is capable of exciting the pixels to produce different colored light.
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The method wherein the OLED device is located with a vacuum or
controlled atmosphere chamber in the manufacturing process.
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The method wherein the OLED device is located on an inspection
station outside the vacuum or controlled atmosphere chamber.
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The method wherein the OLED devices are encapsulated.